Along with the rapid progress of science and technologies, electronic products have been designed to satisfy requirements for light weight, slimness, shortness, and compactness. Recently, in order to meet the requirement for miniaturizing the existing electronic products, various packaging technologies, such as the package-on-package (POP) structure packaging technology, have been developed.
However, during the process of bonding wafers, air bubbles or elevations may be generated in the bonding material. The wafer may be burst or cracked because of thermal expansion and contraction of the bonding material, and the wafer may then be distorted or warped. Hence, how to prevent damages to the wafer or the warp of the wafer through reducing the air bubbles or the elevations in the bonding material has become one of the important research topics.